Wireless communication systems, i.e. systems that provide communication services to wireless communication devices such as mobile phones, smartphones etc. (often denoted by UE that is short for user equipment), have evolved during the last decade into systems that must utilize the radio spectrum in the most efficient manner possible. A reason for this is the ever increasing demand for high speed data communication capabilities in terms of, e.g., bitrate and to provide these capabilities at any given time, at any geographical location and also in scenarios where the wireless communication device is moving at a high speed, e.g., on board a high speed train.
To meet this demand, within the third generation partnership project, 3GPP, work is being done regarding possible enhancements to radio resource management, RRM, performance in high speed train environments. The justification is that there are railways such as Japan Tohoku Shinkansen (running at 320 km/h), German ICE (330 km/h), AGV Italo (400 km/h), and Shanghai Maglev (430 km/h) which vehicles travel at greater than 300 km/h and where there is demand for using mobile services. In a motivation contribution to 3GPP RAN#66, RP-141849, four scenarios of interest to wireless communication network operators are disclosed. In a number of these scenarios, there is a dedicated network to provide railway coverage of the cellular system; either as a standalone network, or used in conjunction with a public network which is not specifically designed to provide high speed train coverage.
Current standard specifications have partly taken UE speeds up to 300 km/h into account, but only in the context of data demodulation, not for cell detection. With increased deployment of high speed train lines, increased number of UE users, and increased usage of bandwidth per user, dominating network operators are requesting improved UE performance and support for speeds exceeding 300 km/h. Future high speed trains are expected to travel at speeds above 500 km/h, e.g. the Superconducting Magnetic Levitation train (SCMaglev) to be deployed in Japan, where train sets have already in April 2015 reached more than 600 km/h in speed tests.
For the development of the fifth generation of mobile telecommunication technology (5G), the International Telecommunication Union (ITU) has defined a set of requirements, IMT-2020, which includes the support of UE speeds of above 500 km/h with respect to mobility and data communication.
Apart from the relatively shortened time for detecting suitable neighbor cells for handover or cell reselection, high speed movement of the UE may also lead to significant Doppler shifts of the received radio signals. Such a Doppler shift forces the UE to increase its demodulation frequency when moving towards a cell (i.e. moving towards an antenna that defines a radio lobe of the cell), and decrease demodulation frequency when moving away from a cell, in order to maintain an acceptable receiver performance.
The Doppler shift can be expressed as:
      Δ    ⁢                  ⁢    f    =      f    ⁡          (                                                  1              +                              v                c                                                    1              -                              v                c                                                    -        1            )      or approximated as
      Δ    ⁢                  ⁢    f    =      f    ⁢          v      c      when v is much smaller than c,where c is the speed of light and v is the relative velocity of the UE towards the transmitting antenna. Referring to FIG. 1, an UE 101 is on a high speed train 103 on a railway track 104, connected to and moving away from cell A2 105 and quickly needs to detect cell B1 107 towards which the UE 101 is moving with a velocity vUE 109 of the train. According to current standard an antenna 111, 113 of a cell site can be as close as 2 m from the railway track 104, mainly motivated by that the wireless communication network would be integrated with the high-speed railway infrastructure. With an angle α between railway track 104 and a direction 106 to a cell antenna 113 and a UE velocity vUE, the relative velocity v towards the transmitting antenna giving rise to Doppler shift is v=vUE cos α.
The magnitude of the Doppler shift depends on the relative velocity of the UE 101 towards the transmitting antenna in a cell. Consequently, with transceivers located close to such a constrained path along which an UE is moving along a railway track, i.e., a small angle between the trajectory of the UE and the line between the UE and the transmitting antenna, a substantial part of the UE velocity will transfer into a Doppler shift. Moreover there will be an abrupt change of sign of the Doppler shift when the UE passes the transmitting antenna and the smaller the angle, the more abrupt is the change from positive to negative Doppler shift.
As part of 3GPP standardization work on improvements on performance in high speed train scenarios it has been proposed a new remote radio head (RRH) deployment for single frequency networks (SFN). This arrangement allows the UE to achieve a good downlink demodulation performance for UEs travelling at speeds up to (at least) 750 km/h.
In the Unidirectional RRH arrangement a UE experiences a nearly constant frequency offset on the downlink, caused by Doppler shift. Whether the frequency offset is positive or negative depends on whether the UE is travelling towards or away from the transmitting RRHs. For the scenario where downlink transmission (DLTX) and uplink reception (ULRX) beams are oriented in the same direction and hence have the same coverage the frequency offset on the uplink (UL) when the signal is received by the ULRX RRH becomes twice that of the one experienced on the downlink. However all UEs onboard the same train will display the same Doppler shift.
However, there remain some challenges in relation to random access prior art in HST scenarios. For example: when there are multiple groups of UEs in the cell with different Doppler characteristics, e.g., trains moving in two directions and/or stationary UEs along the track (passengers on a platform, maintenance workers) there may arise confusion in the PRACH detection since the relative frequency offset between UEs from different groups may be as large as four times the largest Doppler shift. This is considerably larger than can be handled with existing methodology which is based on restricting the set of cyclic shifts for UEs operating in a high speed scenario, as conveyed via an indicator in the PRACH configuration. The confusion arises since PRACH is using a smaller subcarrier distance than other physical channels; 1250 Hz instead of 15000 Hz. With frequency offsets of the received PRACH exceeding about ±1.5 subcarrier distances the received signal may look as another valid PRACH sequence than the one transmitted. This may lead to that a network node is responding to a UE that either doesn't exist or does exist but is not the intended receiver.
A recent proposal in the standardization, captured in 3GPP TS36.878 V12.0.0 section 6.5.3.1, involves a modification of the restricted set to allow even larger frequency offsets, but this approach will only address UEs following 3GPP long term evolution (LTE) Release 13 and onwards. For UEs of earlier releases, which will be dominating for many years to come, the proposal does not help. The problem is that those UEs will use PRACH sequences that have been removed—for a reason—in the new restricted set. Hence there will still be confusion.
In order to facilitate good performance in high speed train scenarios a robust network-based solution is needed for random access when groups of UEs have different Doppler characteristics. The solution needs to provide an acceptable performance for initial access when UE goes from idle to connected mode, and sustained performance for already connected UEs.
In case it is not clear from the context in which they appear, below follows a summary of abbreviations of some of the technical terms used in the description above.
AbbreviationExplanationcDRXConnected mode DRXDLdownlinkDRXDiscontinuous receptionHSTHigh speed trainPRACHPhysical random access channelRARandom accessRARRandom access responseRNTIRadio network temporary identifierRRCRadio resource controlRRHRemote radio headRXReceiveSFNSingle frequency networkTXTransmissionUEuser equipmentULuplink